Gasification is a process that converts carbonaceous materials, such as biomass, into carbon monoxide and hydrogen by reacting the raw material at high temperatures with a controlled amount of oxygen. The resulting gas mixture is called synthetic gas or syngas. Synthetic gas is made predominately of CO (Carbon Monoxide), and Hydrogen. These two elements are the basic building blocks for the Alcohols (Methanol, Ethanol, Propanol, etc.).
Gasification is an efficient method for extracting energy from many different types of organic materials and provides clean waste disposal. Gasification is more efficient than direct combustion of the original fuel, particularly since more of the organics contained in the processed material is converted into energy (higher thermal efficiency).
Syngas may be burned directly in internal combustion engines or used to produce alcohols such as methanol, ethanol and propanol, and also hydrogen. Gasification of fossil fuels is currently widely used on industrial scales to generate electricity.
Typically the generation of synthetic gas in a gasifier goes through several processes.
Pyrolysis
The first process is pyrolysis and this occurs as the temperature inside the gasifying device is raised with an oxygen deprived atmosphere, heating up the carbonaceous material. The pyrolysis process is the gasification of the organics with zero oxygen content. To achieve synthetic gas from the organic material the process could be either a gasification process (partial oxidation of the organic material), or Pyrolysis (zero oxidation of the organic material). Pyrolysis produces more synthetic gas, since it does not oxidize any of the synthetic gas it produces.
Reformer Process
This is effected in a high temperature reformer chamber, which receives the synthetic gases from the pyrolysis chamber. In the reformer chamber the synthetic gas temperature is raised to a high temperature (>900° C.) so as to disassociate the tars into simpler carbon molecules. When steam is added into the reformer chamber the ratio of Hydrogen to Carbon Monoxide is altered, this is achieved via the use of the water gas shift reaction (shift reaction).
The shift reaction is an exothermic chemical reaction in which water and carbon monoxide react to form carbon dioxide and hydrogen:CO+H2O→CO2+H2   (1)
The shift reaction increases the amount of hydrogen produced. However, the shift reaction is an endothermic reaction and requires a high temperature. The shift reaction is sensitive to temperature with the tendency to shift to the products as the temperature increases. As a result, the shift reaction absorbs considerable energy from the reformer chamber, making it cost-prohibitive. Attempts to lower the reaction temperature using catalysts have not been particularly successful.
More importantly, the shift reaction also consumes Carbon monoxide from the synthetic gas. Carbon monoxide is required to produce the require hydrogen to CO ratio for the production of alcohols such as methanol, ethanol and propanol.
There is, therefore, an optimal range for the shift operation, where the use of more shift become less beneficial as both the CO consumption and Energy consumption would be too great.